This invention relates to rotating shaft seals, and more particularly to an advanced stern tube seal for maritime applications.
The current rate of stern tube failure is undesirably high and very costly to the shipping industry. A sudden stern tube failure, for example, in a large oil tanker, can cause a catastrophic financial loss to the owners if it occurs at an inopportune time in the tanker operation. Such as when it is fully loaded and far from port.
The failure of the stern tube lip seal is primarily due to the material incompatibility between the lip seal and the stern tube liner. This incompatibility results in excessive liner and seal wear. In normal lip seals, there are no provisions made for proper lubrication nor for reduction in the sensitivity of the lip seal contact zone to the pressure differential applied across it. Therefore, it is common for the lip seal to fail on fully loaded vessels when the pressure differential across the seal is at its maximum and when the seal failure has the most severe consequence.
The conventional lip seal configuration includes a rubber lip held against the rotating shaft by a garter spring. The garter spring applies a preload to the contact zone between the lip and shaft seal. The aft lip seal normally is subjected to water pressure on one side and an oil pressure on the other side. Ideally, the oil pressure matches the water pressure, and the contact point between the lip and the shaft separates the oil from the water. However, the pressure within the water zone is usually somewhat higher than within the oil zone, and is subject to fluctuation. As the pressure differential across the lip increases, the lip bends and conforms to the shaft. This process is more pronounced in the forward seal where almost the full pressure differential is applied in the form of oil pressure, and the unloaded side of the lip is in the air.
High pressure differences cause an increase in the contact area over which the full pressure differential acts, which results in an increase in the frictional force and an increase in heat generation. This in turn accelerates the wear rate. Excessive wear in the contact zone results in leakage of water into the oil, oil into the water, or oil into the hull depending upon seal location.
Various techniques have been proposed to balance the oil pressure to the water pressure, however, these techniques are usually too sophisticated for the rough environment aboard ship, or are not sensitive enough to follow the pressure fluctuations at the stern tube seals, caused by waves, ship pitching, draft changes, axial vibrations, hull vibrations, etc.